Most fibre reinforced composite components require an outer surface coating to provide an aesthetic and protective finish to the component. Traditionally such components are either painted after moulding or a liquid in-mould coating (gelcoat) with sufficient environmental resistance is used. In some applications painting is preferred, especially when multiple component parts need to be assembled together and any misalignment or joint lines can thereafter be hidden by filling and fairing steps to give a more seamless finish. Painting can also be useful when the final colour has not been defined at the start of the build and the parts can be supplied in a ready to paint format.
A key problem in painting a fibre composite part can be that of preventing the fibre reinforcement pattern appearing in the final surface. This is more of a problem when heavier weight, lower cost reinforcement fibres and fabrics are used to reduce the material cost and the time taken to build up the thickness of the laminate. It is common to use a more expensive lower weight glass fibre layer or a non-structural surfacing tissue in addition to a gelcoat layer to buffer the paint from the fibre reinforcement. It is usual practice to first apply a liquid gelcoat into the mould, which in this case is designed to be easy to sand and repair any defects prior to painting. The gelcoat provides a resin barrier layer between the paint and the first fibre layers by providing a sufficient thickness to stop the fibre pattern showing in the final surface. If the laminate is applied into the mould without the gelcoat barrier coat it is common for the final surface to have pin-hole like defects. Pin-holes are a particular problem when painting as they can be hard to spot on the initial moulding, but when the part is painted, the paint then reticulates to form a larger defect around the pin-hole, requiring rework.
Even when using the gelcoat, it is also the case that sometimes a few pin-holes are present. It would be desirable to have a manufacturing process that substantially completely eliminated the problem of pin-holes.
To apply a gelcoat to larger parts, such as wind turbines, marine craft, architectural mouldings, and bridges additional equipment, such as gelcoat spraying machines and extraction equipment, or mixing equipment used in combination with manual brushing or rolling, is needed to reduce defects and achieve reasonable deposition rates of the gelcoat. A time delay then occurs while waiting for the gelcoat to partially cure to build sufficient strength for the remaining laminate to be added on the mould.
The three main thermoset composite processing methods currently used for manufacturing wind turbine blades are:    1. wet-laminating (also known as open moulding)—in this method, the thermoset resin can cure in ambient conditions, but the tools are usually heated to elevated temperature, 50-90° C., to speed up the resin curing process;    2. the use of pre-preg materials, and the Applicant's own & pre-impregnated dry touch composite material sold under the product name SPRINT®—such materials are typically cured at an elevated temperature between 85° C. to 120° C.; and    3. vacuum assisted resin transfer moulding (also known as VARTM, resin infusion, or vacuum infusion)—in this method liquid resin is infused under a vacuum into a dry fibre composite, and then can cure in ambient conditions, although the tools (i.e. the moulds) are usually heated to an elevated temperature between 50-90° C. to speed up the curing process.
The surface finish quality plays an important role in the aerodynamic efficiency. Some blade manufacturers apply a weather resistant in-mould gelcoat to be the final surface layer, others manufacture spray-paint the blades afterwards. In either case the surface needs to be smooth and defect free. The blade manufacturers currently spend a considerable amount of time filling and fixing the blade surfaces and with the increase demand of wind turbine blades, a solution to decrease the amount of time each blade spends in the finishing production area would save time, reduce cost and increase the production capacity.
VARTM is an attractive process for manufacturing blades due to initial low equipment set-up up cost, improved laminate quality, and health and safety. The usual practice to prepare a VARTM blade for painting is to first apply an in-mould gelcoat coat to give a buffer layer between the paint and the fibre reinforced laminate to prevent cosmetic defects. This in-mould coat then builds in viscosity to form a tacky layer, which is useful to secure the first ply of dry reinforcement fibre as it is difficult to secure dry fabrics against the released tool surface with the usual adhesive spray tack or adhesive tape systems used in the remainder of the fibre stack. On small parts not requiring significant fabric adhesion to the tool, a gelcoat can be replaced by tissues, which become impregnated with resin during the infusion. This is not practical on larger parts which are prone to defects caused by air leaks which tend to accumulate in these layers and even minor air leaks requiring extensive re-work to prepare the component for painting.
Although wet-laminating and VARTM resin systems do cure at ambient conditions in production processes, the tools are often heated to 50-90° C. to speed up the curing process and improve the final mechanical properties. In this case it is possible to combine a catalytic thermoset surface resin film with an ambient curing resin system to achieve a co-cured laminate with a high quality surface finish.
WO 02/094564 discloses a prepreg surface film material which is designed to provide a resin layer which is easy to prepare for painting. However, such pre-preg parts are not suitable for use in resin infusion processes that are widely used to manufacture composite parts.
WO-A-2000/056524 discloses a fibre reinforced composite comprising a fibre reinforcement layer having a first matrix located from a first surface of the fibre reinforcement to a depth only partially through the fibre reinforcement; and a thermoset resin matrix located from a second surface of the fibre reinforcement only partially through the fibre reinforcement and a corresponding method are disclosed. The first matrix material may be a thermoplastic or a thermoset. The first and second matrices may be the same. The fibre reinforcement layer may be fully wetted. Also disclosed is a method of manufacturing a fibre reinforced composite whereby the fibre reinforcement layer and first matrix are enclosed in an envelope; and a second matrix material is injected into the envelope, the second matrix material being a thermoset resin matrix, whereby the second matrix infuses into the second surface of the fibre reinforcement layer. However, this does not disclose how to achieve a high quality surface ready for painting.
There is a need in the art for a fibre-reinforced composite moulding, and method of manufacture thereof, that can readily be painted to achieve a high surface finish without requiring a gelcoat.